A pulse width modulation (PWM) control may be used for controlling brightness, electric power, and driving a motor, etc. For example, in a lighting device that uses a light emitting diode (LED) as a light source that illuminates with a certain color when a current is applied, the brightness of the LED is modulated by pulse width modulation (PWM) control. In other words, the brightness of the LED is modulated by controlling a duty ratio of a period of time during which a current is applied over a period of time during which the current is not applied to the LED. For example, the longer the time during which a current is applied to turn on the LED with respect to the time during which no current is applied to turn off the LED, the brighter the LED becomes. Conversely, the shorter the time during which a current is applied to turn on the LED with respect to time during which no current is applied to turn off the LED, the more the brightness of the LED decreases.
When there is a plurality of controlled objects of PWM control, the plurality of controlled objects are turned on and off substantially simultaneously (for example, a plurality of LEDs are turned on and off). In this case, there is a drawback in that power supply variation takes place.
For example, a plurality of LEDs are used as a light in an automobile vehicle such as a speedometer, an oddmeter, an audio device, a display panel such as for an air conditioner, and a lighting apparatus for an interior light. In this case, when the plurality of LEDs is turned on and off substantially simultaneously, noise due to power supply variation may adversely affect operation of other devices through a power line. Moreover, radiation noise due to the power supply variation may adversely affect reception of radio broadcasting or television broadcasting in an audio device or in a car navigation device.
Thus, conventionally, a plurality of PWM controlled objects is generally divided into a plurality of groups, and the PWM control is applied to each group by shifting the time to start the PWM control for each group. For example, when there are three groups, time to start PWM control is shifted for ⅓ of one period for each group (refer to Japanese Laid-open Patent Publication No. 2008-91311). Accordingly, each group of controlled objects is turned on with different timings.
However, dividing the plurality of PWM controlled objects into groups and starting the PWM control with different timings for each group as described above may sometimes result in overlap of timings when a duty ratio to control each group is different.
Details of a case will be described by referring to FIG. 1 in which PWM control is applied to a first controlled object to a fourth controlled object by outputting a first pulse signal to a fourth pulse signal S1 to S4, respectively.
The four controlled objects are turned on and off according to the four pulses signals S1 to S4. In other words, the four controlled objects are turned on when the four pulse signals S1 to S4 with H level are input. Conversely, the four controlled objects are turned off when the four pulse signals S1 to S4 with L level are input.
For example, a duty ratio of 50% is set for the first pulse signal S1 and the third pulse signal S3, and the first and the third controlled objects are turned on for 50% of one period of the PWM control. A duty ratio of 25% is set for the second pulse signal S2 and the second controlled object is turned on for 25% of one period of the PWM control. Moreover, a duty ratio of 75% is set for the fourth pulse signal S4 and the fourth controlled object is turned on for 75% of one period of the PWM control. Because there are four groups of controlled objects, each group is turned on by shifting ¼ of one period of the PWM control respectively. For example, the first to the fourth controlled objects are turned on at t1, t2, t3, and t4 respectively.
As a result, the first, the second, and the fourth pulse signals, S1, S2, and S4 fall from an H level to an L level substantially simultaneously at time t3. In other words, there is a drawback in which power supply variation may cause, at the same time t3, the first, the second, and the fourth controlled objects to be substantially simultaneously turned off.